In making machinery it is sometimes necessary or desirable to conduct electrical signals between two parts which move relative to each other. Rotary couplings are a common example. For example, some kinds of heavy equipment, such as logging grapples, have a working head which is mounted to the rest of the machine with a rotary joint. The working head typically includes several independent hydraulic circuits which are powered from a hydraulic pump in the body of the machine. It is desirable to control these hydraulic circuits with electrically operated valves mounted in the working head. However, there has been no completely satisfactory means for transmitting electrical control signals through a rotary coupling to the working head of a heavy machine.
In previous heavy machines, each hydraulic circuit has been controlled by valves mounted in the body of the machine. Separate hydraulic lines were then run to and from each hydraulic circuit. This construction generally requires a rotary hydraulic coupler which has two separate fluid connections for each hydraulic circuit. Such rotary hydraulic connectors are expensive to purchase and maintain and are also inconveniently long for many applications. Rotary hydraulic connectors which have many separate hydraulic lines passing through them have a large number of internal seals. If any one seal fails then the entire unit must be serviced.
Where the hydraulic circuits in the working head of a machine are operated by electrically controlled valves in the working head only two hydraulic lines, a pressure line and a return line, are needed. This reduces the cost, size and complexity of the rotary hydraulic coupling. In addition to the hydraulic lines it is necessary to provide electrical power for energizing the electrically controlled valves and signals for controlling the valves in response to commands from the machine's operator. Thus, a rotary electrical coupling is also required.
Most previous rotary electrical couplings comprise a cylindrical drum with several electrically conductive rings extending around its cylindrical outer surface at axially spaced positions. One or more brushes made of graphite, or a similar material, are held in contact with each conductive ring by a spring. The brushes maintain electrical contact with the conductive rings on the drum as the drum rotates.
These previous electrical couplings have several significant disadvantages. The quality of the contact between the brushes and the conductive rings can vary as the drum turns. This can introduce noise into electrical control signals being transmitted through the coupling. Electrical noise can also be introduced if the brushes move when the electrical coupling is subjected to mechanical impact. Electrical noise can prevent control signals from being reliably transmitted through the coupling. These problems can become worse over time. Dust is created as the brushes and conductive rings wear. The dust can interfere with contact between the brushes and the conductive rings and can also cause short circuits between adjacent conductive rings. If the electrical coupling is not extremely well sealed then dust and/or moisture may enter the coupling and make its operation even more unreliable.
These problems are especially severe in heavy equipment which is exposed to frequent mechanical shock and may be operated in very dusty or wet environments.
German patent application No. 4,414,950 discloses a rotary coupling which allows a fluid and electrical signals to be transmitted through the coupling. The coupling is designed primarily for use in painting equipment. The electrical portion of the coupling is sealed from the fluid carrying portion of the coupling.